Conservation of the fortified walls of the Alhambra: Preliminary results on the original and repair materials of the tower of the heads

Abstract

This work shows the preliminary results of the characterisation of earthen materials, rendering mortars and patinas of the Tower of the Heads, belonging to the fortified walls of the Alhambra (Granada, Spain). This work provides a scientifical-technical study of the original and repair building materials of the tower, so as to understand the building techniques used and the causes of deterioration. The tower, indeed, shows severe volume losses and salt deterioration that are badly endangering its conservation. More than thirty samples collected at different heights from the external and internal southern wall and from the external eastern wall of the tower were studied by means of X-ray diffraction (XRD), X-ray fluorescence (XRF), optical microscope observation (OM), high-resolution scanning electron microscopy (HRSEM-EDX) and visual granulometric study (VGS). Cement has been detected in almost all the rendering mortar samples as well as high amounts of gypsum and singenite (K2Ca(SO4)2·H2O). The latter was likely originated as secondary phase from the high-alkaline cement used as binder in the repair mortars. In some original mortars (especially those collected from the interior of the tower) lime and gypsum were found as binders. The great amount of gypsum found in mortars (as both binder and cement additive) explains most of the deterioration processes affecting the tower. It is also worth highlighting that the repair mortars applied for rendering present dolomite as aggregate, which can be deleterious due to the possible formation of Mg-based soluble salts. Finally, the original earthen materials collected from the most inner layers of the wall are composed of high amount of aggregates, mainly quartz and calcite, with lower amounts of phyllosilicates and gypsum. The following step of this research will be linking these results with archaeological and architectural studies on the tower, so as to be able to correctly determine the original building techniques and all the possible interventions carried out on the tower in the past, with the final aim to define a correct intervention strategy that can be used as a model for the rest of towers belonging to the fortified walls of the Alhambra. © 2020, University of Cantabria - Building Technology R&D Group. All rights reserved.